CVE 2017 16943 and CVE 2017 16944 Exim

Background Information

On Friday, November 24 2017, Exim announced two vulnerabilities in versions 4.88 and later.

Impact

According to Exim development: “A remote code execution vulnerability has been reported in Exim, with immediate public disclosure (we were given no private notice).”

The vulnerability exists in the ESMTP CHUNKING extension, and an additional DoS vulnerability exists in the same subsystem. On supported cPanel & WHM versions, chunking_advertise_hosts is set to an un-routable IP address by default. That technique appears to prevent the remote exploitation of the vulnerabilities.

On further investigation, we became concerned that local users may still be able to abuse this configuration. Accordingly, we published an autofixer on Monday, November 27 2017, to fully disable chunking support in Exim. This would have run during Monday’s nightly maintenance, and can be confirmed by running the following as root via SSH: 

/scripts/autorepair exim_disable_chunking

Releases

The following versions of cPanel & WHM were patched to have the correct version of Exim.

  • 68 — 68.0.20

  • 62 — 62.0.36

  • EDGE — 68.0.20

  • CURRENT — 68.0.20

  • RELEASE — 68.0.20

  • STABLE — 68.0.20

How to determine if your server is up to date

The updated RPMs provided by cPanel will contain a changelog entry with the CVE number. You can check for this changelog entry with the following command: 

rpm -q --changelog exim | grep CVE-2017-16943

The output should resemble below: 

- Applied patch for CVE-2017-16943 and CVE-2017-16944

What to do if you are not up to date

If your server is not running one of the above versions, update immediately.

To upgrade your server, use WHM’s Upgrade to Latest Version interface (WHM >> Home >> cPanel >> Upgrade to Latest Version).

Alternatively, you can run the below commands to upgrade your server from the command line: 

1
2

/scripts/upcp
/scripts/check_cpanel_rpms --fix --long-list

Verify the new Exim RPM was installed: 

rpm -q --changelog exim | grep CVE-2017-16943

The output should resemble below: 

- Applied patch for CVE-2017-16943 and CVE-2017-16944

Workarounds

As stated above: you may completely disable chunking support in Exim. To do this, run the following command as root via SSH: 

/scripts/autorepair exim_disable_chunking

Additional Information

  • https://nvd.nist.gov/vuln/detail/CVE-2017-16943

  • https://nvd.nist.gov/vuln/detail/CVE-2017-16944

  • https://lists.exim.org/lurker/message/20171125.034842.d1d75cac.en.html

If you are still experiencing issues or need additional help, contact cPanel support.